The present invention generally relates to heat exchangers and, more specifically, to heat exchangers employed in applications that involve high temperature differentials and high pressures.
Heat exchangers employed in high temperature applications may be subject to various stresses which may cause damage and ultimately failure. For example, high stress heat exchangers may be employed in aircraft to cool bleed air from an engine compressor. In these circumstances, bleed air may emerge from an engine at temperatures in excess of 1000° F. The bleed air may enter a heat exchanger for cooling with ambient air so that the bleed air may be safely utilized in an aircraft environmental control system (ECS). At a typical cruise altitude of an aircraft, ambient air may have a temperature as low as negative 60° F. Thus, various elements of such a heat exchanger may be exposed to an operating temperature differential of almost 1100° F.
In conventional heat exchangers, various elements are joined together with welded or brazed joints. These joints are subjected to thermal stresses when they are exposed to temperature differentials. The joints may also be subjected to stresses when air or fluid is introduced into the heat exchanger at high pressure. Collectively these stresses may cause fatigue-induced failure of the joints. Such failures may cause leakage in the heat exchanger and ultimately may shorten overall life-span of the heat exchanger.
It has been found that tubular type heat exchangers, as compared to plate-fin type heat exchangers, may have a higher tolerance for operating in conditions that produce high pressure and high temperature differentials. On the other hand, tubular type heat exchangers are typically more costly to manufacture and typically have a higher weight than their fin type counterparts.
In some aircraft applications, heat exchangers may be subject to ice formation when an aircraft is allowed to remain idle at ground level in a cold environment. Ice may form on closely spaced fins as water vapor condenses after cessation of airflow through the heat exchanger. When the aircraft is re-started, operation of the heat exchangers must be delayed until the heat exchanger is de-iced.
As can be seen, there is a need for heat exchangers that have a high tolerance for operating under conditions that involve high temperature differentials and/.or high pressures. Moreover there is a need for a plate-fin type heat exchanger that may meet or exceed capabilities of a tubular-type heat exchanger. Further still, there is a need for a heat exchanger that may be rapidly de-iced when employed in an aircraft